This invention relates to a method of producing electric wires (for example, such as insulated cores for communication cables, or high frequency coaxial wires) having an insulating layer of plastic on the periphery of conductor.
The methods of producing foamed plastic insulated wires of the kind heretofore mentioned are divided into two main classes as follows:
1. Chemical foaming method
In this method, a chemical foaming agent and a foaming nucleus are blended into the plastic, and the chemical foaming agent is decomposed during extrusion by heat generated to thereby form gases. This method is very effective and has obtained excellent results to foam low density polyethylenes. The abovementioned method, however, possesses several problems in that it is difficult to form minute bubbles within polypropylene and high density polyethylene. Generally, polypropylene and high density polyethylene require a temperature in the range of from 240.degree. to 280.degree. C. or more when they are extruded. At present, however, it is difficult to find a chemical foaming agent wherein decomposition takes place at such temperatures to thereby form gases.
2. Gas foaming method
The chemical foaming method employs a process wherein the gases are formed in molten plastic by way of thermal decomposition of the foaming agent, whereas the gas foaming method employs a process wherein the gases are directly pressurized and diffused into molten plastic for the purpose of foaming. A simplified process of gas foaming is shown in FIG. 1. As is evident from FIG. 1, when gas is pressurized and directed into molten plastic, a certain amount of the gas is dissolved or diffused in the form of particles into the plastic depending upon temperature, pressure and time. Thereafter, when the gas pressure is released rapidly, the diffused gas particles within the material are over-saturated to form bubbles. At the same time, the plastic is subjected to adiabatic expansion so that the material lowers in temperature and increases in viscosity to prevent the excessive growth of bubbles.
This method may normally be put in practice by using an extruding machine as shown in FIG. 2.
In accordance with the method shown, plastic P and gaseous foaming agent G are fed into a cylinder CY in the extruding machine, the gaseous foaming agent and the plastic are completely blended by means of a screw SC within the cylinder CY to form a uniform molten body, which is then fed under pressure into a die of a cross head H in the extruding machine. When the pressure is released from the die, a foamed insulated layer is coated on the periphery of a wire W travelling in a direction as indicated by the arrow adjacent the cross head. In this case, the pressure within the cylinder of the extruding machine and the pressure within the cross head must be maintained at a high enough pressure to prevent gases in the molten plastic from forming bubbles. Bubble nuclei are formed by release of pressure from the extruding die thereby forming bubbles in the extruded molten plastic, and forming a foamed plastic coating on the wire W.
However, even in the gas foaming method using the extruding machine as described above, there are several difficulties, as noted below, that must be overcome.
(1) Due to unstable extruding pressure, a uniform foamed construction cannot be obtained.
(2) Pour-gas (input gas) is high in pressure, and the diameter of bubbles and the degree of foamation are difficult to control.
(3) In the case where high density polyethylene is the plastic material, the softening temperature (extrusion temperature) is higher than the decomposition temperature of a foaming agent, with the result that a foamed construction is hard to obtain.
(4) It is difficult to maintain the extrusion pressure constant during the entire process. This is particularly a problem during the start of the extrusion operation and the termination of the extrusion operation. As a consequence, a uniform foamed construction cannot be obtained.
It is therefore an object of the present invention to provide a novel and useful method of producing foaming plastic insulated wires while overcoming various problems noted above.